Читать книгу An Introduction to Molecular Biotechnology - Группа авторов - Страница 13

Michael Wink

Heidelberg University, Institute of Pharmacy and Molecular Biotechnology (IPMB), Im Neuenheimer Feld 329, 69120 Heidelberg, Germany

In contrast to the diversity of life forms found in nature with several million species, the cells that make up all of these diverse organisms contain only a limited number of types of inorganic ions and molecules (Table 2.1). Among the most important macromolecules of prokaryotic and eukaryotic cells are polysaccharides, lipids, proteins, and nucleic acids, which are constructed from comparatively few monomeric building blocks (Table 2.2). The membrane lipids (phospholipids, cholesterol) will also be considered in this context because they spontaneously form supramolecular biomembrane structures in the aqueous environment of a cell.

Table 2.1 Molecular composition of cells.

Contents	Bacterium (% of cell mass)	Animal cell (% of cell mass)
Water	70	70
Inorganic ions	1	1
Small molecules (sugars, acids, amino acids)	3	3
Proteins	15	18
RNA	6	1.1
DNA	1	0.25
Phospholipids	2	3
Other lipids	7	2
Polysaccharides	2	2
Cell volume (ml)	2 × 10–12	4 × 10–9
Relative cell volume	1	2000

Table 2.2 Formation and function of the cellular macromolecules.

Basic building blocks	Macromolecule	Function
Simple sugar	Polysaccharide	Structural substances: composition of the cell walls (cellulose, chitin, peptidoglycan); constituents of connective tissues
Storage substances: starch, glycogen
Amino acids	Protein	Enzymes: important catalysts for anabolic and catabolic reaction processes
Hemoglobin: O2 and CO2 transport
Receptors: recognition of external and internal signals
Ion channels, ion pumps, transporters: transport of charged or polar molecules across biological membranes
Regulatory proteins: signal transduction through protein–protein interactions
Transcription regulators: regulation of gene activity
Antibodies: recognition of antigens
Structural proteins: structural organization of supramolecular complexes
Cytoskeleton: formation of molecular networks in the cell that are important for shape and function
Motor proteins: muscle contraction
Phospholipids, cholesterol		Elements of biomembranes
Deoxynucleotide	DNA	Storage, replication, and safe transfer of genetic information; recombination
Nucleotide	RNA	rRNA: structural molecules for the construction of ribosomes
ribozymes and siRNA: catalytic and regulatory processes
tRNA: mediators in translation
mRNA: messengers and mediators between genes and proteins
snRNA: splicing of mRNA
snoRNA: chemically modify rRNA
siRNA: can influence gene expression by directing degradation of selective mRNAs and the establishment of compact chromatin structures
miRNA: can control gene activity,development, and differentiation by specifically blocking translation of particular mRNA
piRNA: bind to piwi proteins and protect germline from transposable elements
lncRNA: apparently play a role in regulating gene transcription

Inorganic ions, sugars, amino acids, fatty acids, organic acids, nucleotides, and various metabolites are counted among the low‐molecular‐weight components and building blocks of the cell. The qualitative composition of cells is similar in prokaryotes and eukaryotes (Table 1.1), even though eukaryote cells generally have a higher protein content and bacterial cells a higher RNA content. Animal cells have a volume that is 10³ times larger than that of bacterial cells.

Owing to their shared evolution, the structure and function of the important cellular molecules is very similar in all organisms, often even identical. Apparently, reliable and functional biomolecules were developed and, if useful for the producer, were selected early in evolution (Table 2.2) and are therefore still used today.